Lance Corporal Matty Hull, killed after an American aircraft opened fire on British tanks. Photograph: Bruce Adams/Daily Mail/PA

The death of Lance Corporal Matty Hull, who was killed by friendly fire when US fighter pilots attacked his convoy in Iraq, was criminally unlawful, a coroner ruled today.

Andrew Walker, the Oxfordshire assistant deputy coroner, hit out at the failure of the US military to cooperate with his investigation, in particular its failure to allow cockpit footage of the incident to be shown in court or to give full details of evidence provided by US air controllers.

Mr Walker said the act was a "criminal one, since the pilots broke with the combat rules of engagement in failing to properly identify the vehicles and seek clearance before opening fire".

He said it would have been easy for the pilot who shot at the convoy to take steps to confirm the identity of the vehicles. In failing to do so he acted "outside the protection of the law of armed conflict".

"I find there was no lawful authority to fire on the convoy. The attack on the convoy therefore amounted to an assault. It was unlawful because there was no lawful reason for it and in that respect it was criminal."

L/Cpl Hull died and four other British soldiers were badly injured when the vehicles in which they were travelling came under fire from the pilots of two American A-10 planes just days after the Iraq invasion in 2003.

His widow, Susan, who has fought a long battle to establish the truth about her husband's death, burst into tears as Mr Walker delivered his verdict. After the ruling, she said it was "very disappointing" that the US president, George Bush, who she met in 2003 and who promised to help her, had not "followed through" on his offer.

"They [British and American forces] were all together serving [in Iraq]. But when it came to following up afterwards I think we were badly let down," Mrs Hull said.

Her lawyer, Geraldine McCool, who also represented families of British servicemen killed by US friendly fire during the 1991 Gulf war, said she had "not seen any evidence" that anything had changed in the intervening period.

The circumstances surrounding how the two part-time reservist pilots came to attack L/Cpl Hull's convoy despite it prominently displaying orange panels to identify itself as friendly have caused tensions between the close allies.

The US military has been accused of attempting to cover up embarrassing mistakes by classifying cockpit recordings of the incident as secret and refusing to make them or the pilots available to the inquest. Key passages of interviews with air controllers have also been kept from the coroner.

The Ministry of Defence said it was "very sorry for confusion and upset" caused over the inquest cockpit footage and vowed to make changes.

"We hope that the inquest has helped them gain a fuller understanding of the circumstances surrounding L/Cpl Hull's death," a spokesman said. "This inquest has highlighted the need for a more coherent approach to the management of documentation and evidence."

He said a dedicated team had been set up to liaise with coroners and bereaved families to ensure relevant documents were made available.

The contents of the cockpit tapes, which the MoD at one point said did not exist, gained a worldwide audience after they were leaked to the Sun newspaper.

The 15-minute recording (which can be seen here as Part 1 and Part 2) clearly shows that the pilots saw the orange markings on the convoy but decided they could have been enemy rocket launchers. When one pilot suggested a return to base, the other said: "I think killing these damn rocket launchers, it would be great."

L/Cpl Hull died three days before his 26th birthday. Two Iraqi civilians, who had been waving a large white flag, were also killed in the attack.

A subsequent US investigation concluded that the pilots, a lieutenant colonel and a major with no combat experience at the time of the incident, were not to blame. In the tapes they were told by controllers that no friendly forces were in their vicinity but were not given permission to open fire.

Major David Small, a spokesman for US central command, last month said the inquiry concluded the incident took place in a complex combat environment; that the pilots believed they were engaging enemy targets, and that they followed the appropriate procedures.